Polarization maintaining (PM) fiber optic single mode couplers are critical components for both interferometric and resonator type fiber-optic gyroscopes (FOG's). In a 2.times.2 PM coupler, light launched along either of the principal axes of a fiber is split into two parts as the output. The quality of a PM coupler is judged by the amount of additional loss it introduces and by its capability to maintain the polarization state launched at the input end. There are two ways of fabricating couplers: mechanical-lapped type, and fused-tapered type. In both the methods aligning of birefringent principal axes of the two fibers forming the coupler is necessary. The mechanical-lapped type involves embedding an unjacketed fiber in a grooved quartz block and mechanically lapping and polishing the block until the fiber core is reached. Two such blocks are bonded together to form a coupler. Low excess loss, high extinction ratio lapped PM couplers have been demonstrated but typically maintain such performance only over a limited temperature range. In addition, lapped coupler fabrication is labor intensive and time consuming making production quite expensive.
The fuse-tapered couplers, on the other hand, are fabricated by fusing a section of two fibers and stretching them until the required power splitting ratio is reached. The fabrication of single mode fused PM couplers involves alignment of birefringent axes of the fibers in addition to fusion and tapering of fibers. Since fusion and tapering produces a single piece of glass in the coupling region, fused couplers generally perform better over environment than do lapped ones. Ease of fabrication also offers significant production cost reductions over lapped couplers.
Maintaining the polarization through the coupling region requires accurate alignment of the fibers prior to fusion and tapering. Any advertently caused misalignment during fusion must be kept low by locally heating the fibers at the line of contact. The key to successfully making fused PM couplers is the apparatus and tooling needed for alignment, fusion and tapering. Also, attainment of high performance PM fused couplers requires the index of the birefringent stress members to be fairly matched with that of the cladding and the use of small (about 80 .mu.m) cladding diameter fiber.
Using the apparatus and tooling, which are described in detail below, packaged fused PM couplers as short as 25 mm in length 2.5 mm in diameter are produced. Typically the extinction ratios of the couplers are better then 20 dB in both the through and coupled output ports, and the excess losses are less than 0.5 dB.